Steam is commonly used in industry as a medium for transporting heat energy from a central location via suitable pipelines to its point of use. The heat energy conveyed in the steam may be released at a suitable point by causing a phase change of the steam from gas to liquid. The phase change releases latent heat from the steam and causes liquid condensate to form.
The condensate and any trapped air needs to be removed from the pipelines, because its presence can cause corrosion and reduce the system's heat transfer efficiency. However, any device for removing condensate also needs to prevent steam from escaping, as this also reduces the system's heat transfer efficiency and may waste the energy transported by that steam.
Traditionally, condensate is removed from pipelines using steam traps. Conventional steam traps comprise moving parts that are arranged to open in the presence of condensate to permit the condensate to drain out of the system and to close in the presence of steam. For example, conventional steam traps may operate mechanically, e.g. using floats and levers. Having parts which move relative to one another in use presents a significant maintenance burden because of an increased risk of failure. Mechanical steam traps may also exhibit limited efficiency, e.g. because of the amount of live steam that is allowed to escape during operation and especially if the steam trap fails when in the open position.
Orifice steam traps are known as an alternative to conventional steam traps. An example of a continuous flow fixed orifice (CFFO) steam trap is described in GB 2 304 300. In such devices, an orifice is located in the flow path of steam travelling through a pipeline. The size of the orifice is chosen carefully such that the condensate, which flows through it much more slowly than the steam, acts to block the steam from passing. The diameter of the hole can be very small, e.g. about 0.4 mm in some cases.
One type of orifice steam trap, known as a venturi orifice trap, has the orifice formed as part of a venturi, i.e. a conduit having a constricted passage therein, the constricted passage having a smaller cross-sectional area then the remainder of the conduit. In venturi orifice traps, the orifice is provided in the constricted part of the venturi, whereby condensate passing through the orifice enters a channel that opens out (e.g. flares outwardly) in the direction of flow. The orifice may thus be provided at the upstream end of a channel having an increasing cross-sectional area. The pressure drop after the orifice causes flash steam to form from the saturated condensate, whereby the venturi restriction is understood to regulate the condensate flow. Such channels have been used in venturi orifice steam trap demonstrated to operate over varying condensate flows without passing live steam. U.S. Pat. No. 4,745,943 describes such an arrangement. Venturi orifice traps are used in systems that exhibit varying loads, e.g. in which the amount of condensate and hence the flow rate of condensate through the trap varies over time (e.g. depending on the level of demand experiences by the heat transfer system).
Venturi orifice traps may be mounted in-line, wherein the venturi is parallel (e.g. coaxial) with the pipeline for conveying the steam. Alternatively, to permit retrofitting and facilitate cleaning of the orifice, it is also known to mount venturi orifice traps on the side of a “quick fit” connector pipe (e.g. a Spirax Sarco pipeline connecter), which itself is mounted in-line. In this arrangement the “quick fit” connector pipe acts to divert the steam to a chamber located out of the pipeline, from which the venturi directs the condensate back into the pipeline (e.g. to a condensate run-off section) in a direction perpendicular to the pipeline. An example of this type of venturi orifice trap is the Emerald Steam Trap manufactured by Gardner Energy Management Limited.
Steam-conveying pipelines used in heat transfer systems as described above contain many steam traps, since the system may have a total pipeline length of several kilometres, and typically a steam trap is required every 30-40 m. For example, an oil refinery may have over 10,000 steam traps distributed along steam pipeline, product trace and jacket heating networks, which may be provided to maintain the temperature of viscous fluids at a level to facilitate pumping.